Method for preparing metal powder

ABSTRACT

A method for preparing a highly crystallized metal powder, comprising: supplying at least one heat-decomposable metal compound powder into a reaction vessel using a carrier gas; and forming a metal powder by heating the metal compound powder in a state in which the metal compound powder is dispersed in a gas phase at a concentration of no more than 10 g/liter, at a temperature that is over the decomposition temperature of the metal compound powder and at least (Tm−200)° C. when the melting point of the metal contained in the metal compound powder is Tm° C. The method provides a high-purity, high-density, highly dispersible, fine, highly crystallized metal powder consisting of spherical particles of uniform size, which is suited to use in thick film pastes, and particularly conductor pastes and the like used in the preparation of multilayer ceramic electronic parts.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for preparing a metalpowder suited to use in electronics, and more particularly to a methodfor preparing a highly crystallized metal powder that is useful as aconductive powder for use in a conductor paste.

[0003] 2. Description of the Related Art

[0004] A conductive metal powder used in a conductor paste for formingan electronic circuit should contain minimized impurities, be a finepowder (with an average particle size from no more than 0.1 μm to about10 μm), have uniform particle shape and size, and consist ofmonodispersed particles with no aggregation. This powder is alsorequired to disperse well in a paste, and have good enough crystallinitythat it will not sinter unevenly. In particular, when the powder is usedto form an internal or external conductor for a multilayer ceramicelectronic part, such as a multilayer capacitor or multilayer inductor,it is required to be a fine spherical, low activity, high crystallinityor single-crystal metal powder which is insusceptible to expansion orshrinking due to oxidation and reduction during firing and highsintering commencement temperature, and which consists of submicronparticles having a finely uniform shape and particle size, in order toprevent delamination, cracks and other structural defects and enable thereduction of the film thickness of a conductor.

[0005] Specifically, a multilayer ceramic electronic part is generallyprepared by alternately laminating a plurality of unfired ceramic greensheet layers of a dielectric material, a magnetic material or the like,and internal conductor paste layers comprising as a conductive componenta powder of a noble metal such as palladium or silver-palladium, or apowder of a base metal such as nickel or copper, and co-firing thelaminated layers at a high temperature. When a base metal that issusceptible to oxidation is used for the internal conductor, variousproblems are encountered. For instance, in the case of using nickelpowder as the conductive component in the internal conductor paste, thelaminated body is heated in an oxidizing atmosphere up to the binderremoval step that is usually carried out at a temperature of about 300to 600° C., so that the organic vehicle in the paste and the ceramicgreen sheets has been completely burned off. During this organic removalstep, nickel is slightly oxidized. Then, firing is performed in an inertatmosphere or a reducing atmosphere, and a reduction treatment isperformed if needed. However, since completely reducing nickel powderthat has been oxidized in the binder removal step is very difficult,deterioration in electrical characteristics, such as an increase inresistance results. Also, these oxidation and reduction are accompaniedby volumetric expansion and shrinkage of the electrodes. Because thesevolumetric changes do not coincide with the sintering shrinkage behaviorof the ceramic layer, delamination, cracking, and other such structuraldefects are apt to occur. Furthermore, a nickel powder quickly sintersin a non-oxidizing atmosphere, and provides a discontinuous film ofinternal conductor due to its oversintering, thereby causing problemssuch as an increased resistivity or internal disruption and resulting inan increased conductor thickness, which is contrary to the need for areduction in thickness of internal conductor layers made along with anincrease in the number of laminated layers in recent years. The aboveoxidation and oversintering have also caused similar problems in thecase where an external conductor is formed by co-firing using a nickelpaste. Therefore, there is a need for a highly crystallized nickelpowder that is resistant to oxidation at least during binder removal andhas a high sintering commencement temperature.

[0006] Meanwhile, palladium, which is a noble metal, has a property thatit is oxidized at a relatively low temperature during firing, and isreduced when further heated to a higher temperature, and this leads tostructural defects caused due to disagreement in sintering and shrinkingbehavior between the conductor layer and the ceramic layer. Therefore,oxidation resistance is desirable with palladium and palladium alloys aswell, and in this respect a spherical, highly crystallized powder issuperior, with a single-crystal powder being particularly good.

[0007] Spray pyrolysis has heretofore been known as a conventionalmethod for preparing a well-crystallized metal powder such as this. Asdiscussed in Japanese Patent Publication 63-31522 and elsewhere, spraypyrolysis is a method in which a solution containing one or more metalcompounds, or a suspension in which these compounds have been dispersed,is sprayed in the form of fine droplets, and these droplets are heatedat a temperature higher than the decomposition temperature of the metalcompounds, and preferably a temperature near or above the melting pointsof the metals contained in the metal compounds, so that the metalcompounds are pyrolyzed and metal or alloy powder is precipitated. Thismethod yields a highly crystallized or single-crystal, high-density,highly dispersible, true-spherical metal powder or alloy powder. Unlikea wet-type reducing method, preparation is easy because there is no needfor solid-liquid separation, and since no additives or solvents thatwould affect the purity of the product are used, an advantage is that ahigh-purity powder containing no impurities is obtained. Furthermore, itis easy to control the particle size, and since the composition of theproduced particles is basically the same as the composition of thestarting metal compounds in the solution, it is also easy to control thecomposition of the produced particles.

[0008] A problem encountered with this method, however, is that becausewater or an organic solvent such as alcohol, acetone, or ether is usedas a dispersion medium or solvent for making droplets out of the metalcompound raw material, there is considerable energy loss duringpyrolysis, and the cost is high. Specifically, in this method, pyrolysisof the metal compound is carried out simultaneously with the evaporationof the solvent by heating, or pyrolysis of the metal compound is carriedout after the evaporation of the solvent, but in either case atremendous amount of energy is needed to evaporate the solvent. Also,the droplets are subjected to aggregation or disruption inside thereaction vessel, which results in a broad particle size distribution inthe resulting powder. This makes it difficult to set the spray velocity,the droplet concentration in the carrier gas, the dwelling time in thereaction vessel, and the reaction conditions. Particularly, with apowder of a base metal such as nickel, iron, cobalt, or copper, thepyrolysis must be conducted in a carefully controlled reducing or weaklyreducing atmosphere in order to prevent oxidation. Furthermore, whenwater is used as a solvent, oxidation tends to occur at high temperaturebecause of the oxidizing gas produced by the decomposition of the water,and this makes it difficult to obtain a powder with good crystallinity.

[0009] A method for preparing metal ultra fine particles by a vaporphase process is also well known. For instance, to obtain a nickelpowder, nickel chloride is vaporized and then reduced by a reducing gasat a high temperature. However, a powder obtained by a precipitationreaction from the vapor phase is prone to aggregation, and furthermoreit is difficult to control its particle size. It is also impossible toproduce an alloy of metals with different vaporization pressures in anaccurately controlled composition.

[0010] U.S. Pat. No. 5,976,217 discloses a method for reducing metalcompound powder, such as tungsten oxide powder, by a solid-gas reactionusing a reducing agent. This method comprises introducing a metalcompound powder material along with a reducing gas and optionally acarrier gas into a heated reaction chamber, and passing them the chamberalong the predetermined paths, wherein the material is subjected tochemical reduction. The reduction of the material to be reduced iscaused in contact with the reducing gas. Therefore, when the material isprovided in a solid powder state, the contact area between the materialand the reducing gas is small as compared with the above-mentioned vaporphase process and therefore it is difficult to complete the reduction ofthe powdered material to metal in a short time. Even if a cyclone isused to ensure a longer path or cause explosion of the materialparticles, unreacted or incompletely reduced material still remains,making it difficult to set the suitable processing parameters (e.g., thetravelling path, the way of feeding the reducing gas, etc.). Further, inthis patent there is no mention about obtaining a uniform sphericalmetal powder with good crystallinity suited to use in electronics.

[0011] Japanese Patent Publication 36-9163 discloses a method forpreparing a high-purity metal powder or mixture thereof by pyrolyzing asilver, nickel, or copper salt of an organic polybasic acid in air or aninert gas at a relatively low temperature, namely, 100 to 500° C., andalso states that a fine metal powder with a particle size of just a fewmicrons or less can be obtained by grinding. With this method, though,not only is it impossible to control the particle size precisely, but ifthe material is heated to close to or over the melting point of themetal in an effort to raise crystallinity, the particle shape cannot bemaintained, and the particle size cannot be brought to just a fewmicrons or less by grinding.

SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to obtain a high-purity,high-density, highly dispersible, fine, highly crystallized metal powderconsisting of spherical particles of uniform size, which is suited touse in thick film pastes, and particularly conductor pastes and the likeused in the preparation of multilayer ceramic electronic parts. It isanother object of the present invention to provide a novel method forpreparing a metal powder such as this by a low-cost and simpleprocedure.

[0013] Specifically, the present invention resides in a method forpreparing a highly crystallized metal powder, comprising:

[0014] supplying at least one heat-decomposable metal compound powderinto a reaction vessel using a carrier gas; and

[0015] forming a metal powder by heating the metal compound powder in astate in which the metal compound powder is dispersed in the gas phaseat a concentration of no more than 10 g/liter, at a temperature that isover the decomposition temperature of the metal compound powder and atleast (Tm−200)° C. when the melting point of the metal contained in themetal compound powder is Tm° C.

[0016] The present invention also resides in the above-mentioned methodfor preparing a highly crystallized metal powder, wherein the metalcompound powder is a homogeneous mixed powder or a composite powder of ametal compound or compounds including two or more metal elements, andthe metal powder is an alloy powder.

[0017] The present invention also resides in a highly crystallized metalpowder prepared by the above-mentioned method, a conductor pasteincluding this highly crystallized metal powder, and a multilayerceramic electronic part in which a conductor layer is formed using theabove-mentioned conductor paste.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] There are no particular restrictions on the metal powder preparedwith this method, but this method is particularly well suited to theprepare of a powder of a base metal such as copper, nickel, cobalt, oriron, or of a noble metal powder such as silver, palladium, gold, orplatinum. A mixed powder or alloy powder comprising a plurality ofmetals can be obtained through appropriate selection of the raw materialmetal compound powder or powders. The term “metal powder” as used in thepresent invention encompasses these mixed powders and alloy powders.

[0019] The heat-decomposable metal compound powder used as the rawmaterial of the metal powder can be an inorganic compound, such as ahydroxide, nitrate, sulfate, carbonate, oxynitrate, oxysulfate,chloride, oxide, ammonium complex, or phosphate, or it can be an organiccompound, such as a carboxylate, resinate, sulfonate, acetylacetonate,or metal mono- or polyhydric alcoholate. A hydroxide, carbonate,carboxylate, resinate, acetylacetonate, alcoholate, or the like ispreferable here because there will be no harmful by-products afterpyrolysis.

[0020] A mixture of two or more metal compound powders can also be usedas the raw material powder.

[0021] When an alloy powder is prepared, raw material powders for thealloy component metals may simply be uniformly mixed in the requiredcompositional ratio, but it is preferable to use a composite powder inwhich a plurality of metal components have already been compounded so asto be included in the specified compositional ratio in each particle ofthe raw material powder. This compounding can be accomplished by amethod in which the metal compound powders that serve as the rawmaterials are mixed ahead of time, and then heat treated until thecomposition is uniform, and then pulverized, or a known method such as asol-gel method or co-precipitation method. A double salt, complex salt,metal double oxide, or the like may also be used.

[0022] In the method of this invention, it is assumed that one metal oralloy particle is formed from one particle of the raw materialheat-decomposable metal compound. Therefore, the size of the resultingmetal particles is substantially proportional to the particle size ofthe raw material powder although the proportion thereof depends on thekind of the metal compound. Therefore, to obtain a metal powder with auniform particle size, a metal compound powder with a uniform particlesize is used. If the particle size distribution of the raw materialpowder is wide, it is preferable to adjust the particle size bypulverization, grinding, or classification with a pulverizer orclassifier prior to dispersing the material powder in the gas phase.

[0023] It is important in the present invention that the solid metalcompound powder be pyrolyzed in a state of being dispersed in the gasphase. Furthermore, inside the reaction vessel, the raw material powdermust be pyrolyzed in a state of being dispersed at a concentration lowenough that particles of the raw material powder and particles of theproduct powder do not collide with each other. Accordingly, theconcentration of the metal compound in the gas phase must be 10 g/literor less. If the concentration is above this, collisions between theparticles will prevent a metal powder with a uniform particle size frombeing obtained. The dispersion concentration may be appropriatelydetermined depending on the type of the dispersing device or thepyrolyzing device. There are no particular restrictions on theconcentration, as long as it is 10 g/liter or less. However, if themetal compound concentration is too low in the gas phase, productionefficiency will also be low. Therefore, at least 0.01 g/liter is usuallypreferable.

[0024] There are no particular restrictions on the means for dispersingthe raw material powder in the gas phase, and any ordinary dispersingdevice can be used. In order to conduct the pyrolysis while maintainingthe above-mentioned low concentration dispersion state, for example, atubular reaction vessel heated from the outside is used, the rawmaterial powder is supplied from one opening along with a carrier gas ata constant flow rate and made to pass through the reaction vessel, andthe metal powder produced by pyrolysis is recovered at the otheropening. The flow rate of a mixture of the raw material powder and thecarrier gas and the time for the mixture to pass through the reactionvessel is set depending on the devices used so that the powder will befully heated at the desired temperature. The heating may be from theoutside of the reaction vessel, using an electric furnace, gas furnace,or the like, or a fuel gas may be supplied to the reaction vessel to usethe combustion flame thereof.

[0025] In the case of preparing a noble metal powder, there are noparticular restrictions on the carrier gas, and an oxidizing gas (e.g.,air, oxygen, water vapor, etc.), an inert gas (e.g., nitrogen, argon,etc.), or a mixture of these can be used. In the case of preparing abase metal such as nickel or copper, which is susceptible to oxidation,an inert gas is used and such an inert gas may be used in combinationwith a reducing gas, such as hydrogen, carbon monoxide, or methane, inorder to carry the pyrolysis in a somewhat reducing atmosphere and tothereby effectively preventing the oxidation of the metal powder.

[0026] One of the characteristics of the present invention is that thereis no need for precise adjustment of the atmosphere during heating.Especially, in the case of base metal, if the raw material is a metalcompound capable of producing carbon monoxide, hydrogen, methane, or thelike when pyrolyzed in an inert gas, and thereby creating a reducingatmosphere, then a metal powder with almost no oxidation will beobtained without any need to supply a reducing gas from the outside tothe reaction system. For example, when a nickel powder is prepared byconventional spray pyrolysis using an aqueous solution, a reducing gasusually must be introduced in a precisely controlled amount in order toprevent the oxidation of the nickel. With the method of the presentinvention, however, when nickel acetate or another carboxylate powder isused for the raw material and pyrolysis is conducted in a nitrogenatmosphere, for example, the decomposition of the carboxylic acidradicals generates carbon monoxide and hydrogen, and the inside of thereactor becomes a reducing atmosphere, so a nickel powder with almost nooxidation is obtained.

[0027] The metal powder obtained with the method of the presentinvention consists of spherical primary particles of uniform size andwith no aggregation. Also, the crystallinity is good, there are fewdefects inside the particles, and substantially no grain boundaries areincluded. This means that activity is low even though the powder is veryfine. In particular, even readily oxidizable metals such as nickel,iron, cobalt, copper, and other such base metals or palladium can bestored stably in air because of their insusceptibility to oxidation, andthis oxidation resistance is preserved even at high temperatures.Therefore, when the powder is used in a conductor paste for an internalor external conductor of a multilayer capacitor, there will be noincrease in resistance due to the oxidation of the conductive metal, andno delamination, cracking or other such structural defects attributableto oxidation and reduction during firing, thereby allowing a capacitorwith outstanding characteristics to be prepared.

[0028] If the melting point of the target metal or alloy is Tm° C., aspherical highly crystallized metal powder will not be obtained if theheating temperature is lower than (Tm−200)° C. In particular, in orderto obtain true-spherical single-crystal metal powder particles with asmooth surface, it is preferable for the heating to be performed at orabove the melting point of the target metal or alloy. If oxides,nitrides, carbides, or the like are produced by this metal during orafter the pyrolysis, then the heating must be performed under theconditions causing the decomposition of these oxides, nitrides, orcarbides.

[0029] The present invention will now be described in specific termsthrough examples and comparative examples.

EXAMPLE 1

[0030] A powder of nickel acetate tetrahydrate was supplied to a jetmill at a feed rate of 5 kg/hr, and was pulverized and dispersed bynitrogen gas flowing at a rate of 200 liter/min. The thus obtainedpowder had an average particle size of 1.0 μm and a maximum particlesize of 3.0 μm. The concentration of the nickel acetate tetrahydratepowder in the gas phase was 0.4 g/liter. At this power concentration,the gas-solid mixture thus obtained was introduced into a reaction tubeinside an electric furnace heated to 1550° C., where it was heated anddecomposed, and the produced powder was collected using a bag filter.

[0031] The powder thus obtained was analyzed by an X-ray diffractometer,which revealed it to be a metallic nickel single-crystal powder. Thispowder was also observed under a scanning electron microscope (SEM),which revealed the powder to consist of true-spherical particles thatwere free from aggregation, with an average particle size of 0.5 μm anda maximum particle size of 2.0 μm. Thermogravimetric analysis wasperformed in air, but no oxidation occurred up to 350° C. Apolycrystalline nickel powder with an average particle size of 0.5 μmobtained by a wet method has an oxidation temperature of 250° C., so itcan be seen that the nickel powder of the present invention is a stablepowder.

EXAMPLES 2 AND 3

[0032] Nickel powders were prepared in the same manner as in Example 1,except that the temperature of the electric furnace was changed to 1300°C. and 1650° C., respectively. Table 1 shows the characteristics of thepowders thus obtained.

EXAMPLE 4

[0033] A nickel powder was prepared in the same manner as in Example 1,except that the feed rate of the nickel acetate tetrahydrate powder intothe jet mill was changed to 62.5 kg/hr., the nickel acetate tetrahydratepowder introduced into the reaction tube had an average particle size ofabout 2.5 μm and a maximum particle size of about 6.0 μm, and the nickelacetate tetrahydrate powder concentration in the gas phase was 5.0g/liter. Table 1 shows the characteristics of the powder thus obtained.

EXAMPLES 5 AND 6

[0034] Nickel powders were prepared in the same manner as in Example 1,except that nickel formate dihydrate powder and nickel oxalate dihydratepowder were used, respectively, in place of the nickel acetatetetrahydrate powder. Table 1 shows the characteristics of the powdersthus obtained.

COMPARATIVE EXAMPLE 1

[0035] A nickel powder was prepared in the same manner as in Example 1,except that the feed rate of the nickel acetate tetrahydrate powder intothe jet mill was changed to 150 kg/hr., the nickel acetate tetrahydratepowder introduced into the reaction tube had an average particle size ofabout 5.0 μm, and the nickel acetate tetrahydrate powder concentrationin the gas phase was 12.0 g/liter. The powder thus obtained was observedby SEM, which revealed that numerous particles of high crystallinity hadfused together into huge particles of irregular shape, and the particlesize distribution was wide.

COMPARATIVE EXAMPLE 2

[0036] A nickel powder was prepared in the same manner as in Example 1,except that the temperature of the electric furnace was changed to 1100°C. The powder thus obtained had an irregular shape and had a wideparticle size distribution, as shown in Table 1, and was an agglomerateof microcrystals, in which the crystallinity was low. Oxidationresistance was also low.

EXAMPLE 7

[0037] A nickel acetate tetrahydrate powder and a copper acetate powderwere mixed in advance such that the weight ratio of the metal componentswas Ni:Cu=7:3, and a powder was prepared from this mixture by the samemethod as in Example 1.

[0038] The powder thus obtained was examined by X-ray diffraction, whichrevealed it to be a single-crystal nickel-copper alloy. Table 1 showsthe characteristics.

EXAMPLE 8

[0039] A powder was prepared by the same method as in Example 1, exceptthat a powder of palladium chloride was used as the raw material, theconcentration of the powder dispersed in the gas phase was changed to1.0 g/liter, air was used as the pulverization gas and the carrier gas,and the temperature of the electric furnace was changed to 1600° C.

[0040] The powder thus obtained was examined by X-ray diffraction, whichrevealed it to be a single-crystal powder of metallic palladium. Table 1shows the characteristics.

EXAMPLE 9

[0041] A palladium chloride powder and a silver acetate powder weremixed in advance such that the weight ratio of the metal components wasPd:Ag=2:8, and a palladium-silver alloy single-crystal powder wasprepared from this mixture by the same method as in Example 8, exceptthat the concentration of the powder dispersed in the gas phase waschanged to 0.4 g/liter and the temperature of the electric furnace waschanged to 1400° C. Table 1 shows the characteristics.

COMPARATIVE EXAMPLE 3

[0042] A powder was prepared by the same method as in Example 9, exceptthat the temperature of the electric furnace was changed to 900° C. Asshown in Table 1, the powder thus obtained was a mixture of a palladiumoxide powder and a palladium-silver alloy powder with low crystallinity.

[0043] It can be seen from the results of Example 9 and ComparativeExample 3 that oxidation resistance is far superior with thepalladium-silver alloy powder obtained with the present invention. TABLE1 Characteristics of produced powder Metal compound Oxidation powderHeating Average Maximum commence- Raw material concentration temper-Compo- particle particle ment temp. Crystal- powder (g/liter) ature (°C.) sition size (μm) size (μm) (° C.) Shape linity Example 1 nickelacetate 0.4 1550 Ni 0.5 2.0 350 true single tetrahydrate sphericalcrystal Example 2 nickel acetate 0.4 1300 Ni 0.6 2.0 330 polyhedralhighly tetrahydrate spherical crystallized Example 3 nickel acetate 0.41650 Ni 0.5 1.8 360 true single tetrahydrate spherical crystal Example 4nickel acetate 5.0 1550 Ni 1.5 3.5 400 true single tetrahydratespherical crystal Example 5 nickel formate 0.4 1550 Ni 0.8 2.2 340 truesingle dihydrate spherical crystal Example 6 nickel oxalate 0.4 1550 Ni0.4 1.3 320 true single dihydrate spherical crystal Comparative nickelacetate 12.0 1550 Ni ≧10 Unmeasurable 400 irregular highly Example 1tetrahydrate crystallized Comparative nickel acetate 0.4 1100 Ni 1.8 4.8250 irregular micro Example 2 tetrahydrate crystalline Example 7 nickelacetate 0.4 1550 Ni/Cu 0.6 2.0 330 true single tetrahydrate, alloyspherical crystal copper acetate Example 8 palladium 1.0 1600 Pd 0.6 1.8600 true single chloride spherical crystal Example 9 palladium 0.4 1400Ag/Pd 0.6 1.3 Not true single chloride, silver alloy oxidized sphericalcrystal acetate Comparative palladium 0.4 900 Ag/Pd 0.9 2.5 350irregular micro- Example 3 chloride, silver alloy, crystalline acetatePdO

[0044] In accordance with the method of the present invention, a metalpowder that is spherical and has good crystallinity and gooddispersiblity can be obtained with ease. Also, it is possible to obtaina single-crystal metal powder by heating the metal compound as a sourcematerial to a temperature at or above the melting point of the metalcontained in the metal compound. Since no additives or solvents thatwould affect purity are used, a high-purity powder containing noimpurities is obtained.

[0045] Furthermore, this method allows a metal powder to be obtainedwith a uniform particle size by controlling the particle size of the rawmaterial powder, so adjusting the particle size is also easy. Therefore,there is no need for a classification step, and an extremely fine powderwith a narrow particle size distribution that is suited to use in athick film paste can be obtained.

[0046] Since the raw material is not in the form of a solution orsuspension, there is less energy loss caused by solvent evaporation thanwith an ordinary spray pyrolysis method, allowing the powder to beprepared easily and at low cost. Furthermore, there is no problem withfusion of the droplets, and these droplets can be dispersed in the gasphase at a relatively high concentration, so efficiency is better.

[0047] In addition, since no oxidizing gas is generated from a solvent,this method is suited to the preparation of base metal powders that areprone to oxidation and require synthesis under a low oxygen partialpressure. Furthermore, if the compound is selected properly, oxidationcan be minimized without requiring feeding a reducing gas from theoutside, so the reaction conditions are easier to set. Finally, theobtained metal powder has low activity and good oxidation resistance,and therefore when it is used in a conductor paste for forming aconductor for a multilayer capacitor or the like, it is possible toprepare parts that have no delamination, cracks or other structuredefects and are therefore highly reliable.

What is claimed is:
 1. A method for preparing a highly crystallizedmetal powder, comprising: supplying at least one heat-decomposable metalcompound powder into a reaction vessel using a carrier gas; and forminga metal powder by heating the metal compound powder in a state in whichthe metal compound powder is dispersed in a gas phase at a concentrationof no more than 10 g/liter, at a temperature that is over thedecomposition temperature of the metal compound powder and at least(Tm−200)° C. when the melting point of the metal contained in the metalcompound powder is Tm° C.
 2. The method for preparing a highlycrystallized metal powder according to claim 1, wherein the metalcompound powder is a homogeneous mixed powder or a composite powder of ametal compound or compounds including two or more metal elements, andthe metal powder is an alloy powder.
 3. The method for preparing ahighly crystallized metal powder according to claim 1, wherein thecarrier gas is an inert gas, and the metal powder is nickel powder,copper powder, or an alloy powder containing nickel and/or copper. 4.The method for preparing a highly crystallized metal powder according toclaim 1, wherein a metal compound powder which brings the atmosphereduring pyrolysis to a reducing atmosphere by being pyrolyzed in an inertgas is used as the metal compound powder.
 5. The method for preparing ahighly crystallized metal powder according to claim 4, wherein the metalcompound powder is a metal carboxylate powder.
 6. The method forpreparing a highly crystallized metal powder according to claim 1,wherein the metal powder is a palladium powder or an alloy powdercontaining palladium.
 7. A highly crystallized metal powder prepared bythe method according to claim
 1. 8. A conductor paste containing thehighly crystallized metal powder according to claim
 7. 9. A multilayerceramic electronic part in which a conductor layer is formed using theconductor paste according to claim 8.